With the spread of electronic appliances including office automation apparatuses and communication instruments, electromagnetic wave emission from these appliances have come into a problem. That is, adverse effect of electromagnetic wave to the human body is feared and it is also a problem that the electromagnetic wave affects precision apparatus to cause malfunction.
Therefore, plates having good electromagnetic-wave shielding efficiency and light transparency have developed as front filters for PDPs of the office automation apparatuses and come into commercial use. Such plates are also used as windows of a place where a precision apparatus is installed, such as a hospital or a laboratory in order to protect the precision apparatus from electromagnetic waves form a portable telephone.
A conventional electromagnetic-wave shielding and light transmitting plate typically comprises transparent base plates such as acrylic boards and a conductive mesh member like a wire netting and is formed by interposing the conductive mesh member between the transparent base plates and by assembling them.
A conductive mesh member conventionally used for an electromagnetic-wave shielding and light transmitting plate generally has a wire diameter between 10 .mu.m and 500 .mu.m, a sieve opening from about 5 meshes to about 500 meshes, and an open area ratio less than 75%.
In order to provide good electromagnetic-wave shielding efficiency when such an electromagnetic-wave shielding and light transmitting plate is assembled in a body of equipment such as PDP, it is necessary to provide uniform current conduction between the electromagnetic-wave shielding and light transmitting plate and the body of equipment, that is, between the conductive mesh of the electromagnetic-wave shielding and light transmitting plate and a conduction surface of the body.
A structure, which can provide good current conduction between an electromagnetic-wave shielding and light transmitting plate and a body of equipment but is simple, has conventionally proposed (JPA 9-147752). This structure is made by forming a conductive mesh member in such a size that the periphery thereof is positioned outside of peripheral edges of transparent base plates so as to form margins when it is interposed therebetween, then folding the margins on the surface of one of the transparent base plates so that the margins function as conductive portions between the electromagnetic-wave shielding and light transmitting plate and the body of equipment, and bonding the margins to the body of equipment by pressure bonding.
As for the conductive mesh member conventionally used, the larger the wire diameter, the larger is the sieve opening, or, the smaller the wire diameter, the smaller is the sieve opening. This is because a mesh having larger sieve opening can be made by using fibers of larger wire diameter, but it is quite difficult to make a mesh having larger sieve opening by using fibers of smaller wire diameter.
Accordingly, conventional electromagnetic-wave shielding and light transmitting plates employing such conductive mesh member are at a disadvantage in that the light transparency obtained by even the best one is around 70%, that is, good light transparency can not be obtained.
The conventional electromagnetic-wave shielding transmitting plates are also at a disadvantage in that moire phenomenon (interference fringes) easily occurs because of the relation between the conductive mesh and pitch of pixels of a light emitting panel on which the electromagnetic-wave shielding and light transmitting plate is mounted.
Though both light transparency and electromagnetic-wave shielding efficiency can be provided by combination of such a conductive mesh member and a transparent conductive film, the transparent conductive film is at a disadvantage in that it is not easy to provide conduction relative to a body of equipment.
The conductive mesh member can provide conduction relative to the body of the equipment by means of forming the conductive mesh member such that its periphery is positioned outside of peripheral edges of a transparent base plate and folding the peripheral portions of the conductive mesh member along the peripheral edges of transparent base plate. That is, the conduction between the conductive mesh member and the body of the equipment can be obtained through the folded peripheral portions of the conductive mesh member. In case of the transparent conductive film, however, if the latter is formed so that its periphery is positioned outside of peripheral edges of the transparent base plate and then folded along the peripheral edges of the transparent base plate, the film may tear at the folded portions so that the conduction between the film and the transparent base plate can not be obtained.
Instead of the transparent conductive film, a transparent conductive layer may be directly formed on a bonding surface of one of transparent base plates. In this case, however, the transparent conductive layer is covered by the other transparent base plate, so the conduction from the transparent conductive layer to the body of equipment can not be obtained.
In case of employing a transparent conductive film, any design change, such as forming a through hole in a transparent base plate to provide conduction relative to the transparent conductive film, is necessary, thereby making complex the assembly of an electromagnetic-wave shielding and light transmitting plate and the operation for building in the body of the equipment.
On the other hand, a PDP utilizing a discharging phenomenon has the following advantages in comparison to a liquid crystal display (LCD) and a cathode ray tube (CRT). Therefore, recently it has been researched and developed for practical use, for example, televisions, office automatic apparatus such as personal computers and word processors, traffic apparatus, boards, and other kinds of display panels.
1. It utilizes discharge light so that it is spontaneous light. PA1 2. As its discharge gap is 0.1-0.3 mm, it can be shaped in panel. PA1 3. By using fluorescent substances, it can emit colors. PA1 4. It eases to make wide screen. PA1 1. Structure for disposing two panels is complicated. PA1 2. As a transparent base plate made of glass or the like is required for each of the PDP and the electromagnetic-wave shielding transparent plate, the PDP and the electromagnetic-wave shielding transparent plate make thicker and heavier in total. PA1 3. The number of parts and man-hours are increased, thereby raising the cost.
The basic display mechanism of the PDP is displaying of letters and figures by selective discharge emitting of fluorescent substances in many discharge cells which are disposed distantly each other between two plate glasses, and for example, has a mechanism as shown in FIG. 6.
In FIG. 6, a numeral 61 designates a front glass, 62 designates a rear glass, 63 designates a bulkhead, 64 designates a display cell (discharge cell), 65 designates an auxiliary cell, 66 designates a cathode, 67 designates a display anode, 68 designates an auxiliary anode. A red fluorescent substance, a green fluorescent substance, or a blue fluorescent substance (not shown) is provided in a film form on internal walls of each display cell 64 and these fluorescent substances emit light by electrical discharges when a voltage is applied between electrodes.
From the front surface of the PDP, electromagnetic waves with frequency from several kHz to several GHz are generated due to applying voltage, electrical discharge, and light emission, and the electromagnetic waves have to be shielded. Moreover, for improving its display contrast, reflection of external light at the front surface has to be prevented.
In order to shield such electromagnetic waves from PDP, a transparent plate which has electromagnetic-wave shielding efficiency is disposed in front of the PDP.
In this case, conductive mesh member conventionally used for an electromagnetic-wave shielding and light transmitting plate generally has a wire diameter between 10 .mu.m and 500 .mu.m, a sieve opening from about 5 meshes to about 500 meshes, and an open area ratio less than 75%. As for the conductive mesh member conventionally used, the larger the wire diameter, the larger is the sieve opening, or, the smaller the wire diameter, the smaller is the sieve opening. This is because a mesh having larger sieve opening can be made up of fibers of larger wire diameter, but it is quite difficult to make a mesh having larger sieve opening with fibers of smaller wire diameter.
The PDP which the separate transparent plate is disposed in front of the PDP has defects as follows:
Conventional electromagnetic-wave shielding and light transmitting plates employing the conductive mesh member which used, the larger the wire diameter, the larger is the sieve opening, or, the smaller the wire diameter, the smaller is the sieve opening, are at a disadvantage in that the light transparency obtained by even the best one is around 70%, that is, good light transparency can not be obtained.
The conventional electromagnetic-wave shielding transmitting plates are also at a disadvantage in that moire phenomenon (interference fringes) easily occurs because of the relation between the conductive mesh and pitch of pixels of a light emitting panel.
As mentioned above, a conventional electromagnetic-wave shielding and light transmitting plate typically comprises transparent base plates such as acrylic boards and a conductive mesh member like a wire netting and is formed by interposing the conductive mesh member between the transparent base plates and by assembling them.
In order to provide good electromagnetic-wave shielding efficiency when such an electromagnetic-wave shielding and light transmitting plate is assembled in a body of equipment such as PDP, it is necessary to provide uniform current conduction between the electromagnetic-wave shielding and light transmitting plate and the body of equipment, that is, between the conductive mesh of the electromagnetic-wave shielding and light transmitting plate and a conduction surface of the body.